1. The Field of the Invention
The present invention relates to sheets made from moldable hydraulically settable materials, which can be used in making containers, printed materials, and other objects. More particularly, the present invention relates to compositions and methods for readily and economically forming a hydraulically settable mixture into relatively thin, flexible sheets that can be cured and then stored in a roll or flat sheets until needed, or scored and folded into the desired container, printed material, or other object. The hydraulically settable sheets can be used much like paper or cardboard, plastic or polystyrene, and metal materials and can be readily moldable into the desired container or object.
2. The Relevant Technology
A. Traditional Hydraulically Settable Materials.
Hydraulically settable materials such as those that contain hydraulic cement or gypsum (hereinafter "hydraulically settable," "hydraulic," or "cementitious" compositions, materials, or mixtures) have been used for thousands of years to create useful, generally large, bulky structures that are durable, strong, and relatively inexpensive. For example, cement is a hydraulically settable binder derived from clay and limestone, and it is essentially nondepletable.
Those materials containing a hydraulic cement are generally formed by mixing hydraulic cement with water and usually some type of aggregate to form a cementitious mixture, which hardens into concrete. Ideally, a freshly mixed cementitious mixture is fairly nonviscous, semi-fluid, and capable of being mixed and formed by hand. Because of its fluid nature, concrete is generally shaped by being poured into a mold, worked to eliminate large air pockets, and allowed to harden. If the surface of the concrete structure is to be exposed, such as on a concrete sidewalk, additional efforts are made to finish the surface to make it more functional and to give it the desired surface characteristics.
Due to the high level of fluidity required for typical cementitious mixtures to have adequate workability, the uses of concrete and other hydraulically settable mixtures have been limited mainly to simple shapes which are generally large, heavy, and bulky, and which require mechanical forces to retain their shape for an extended period of time until sufficient hardening of the material has occurred. Another aspect of the limitations of traditional cementitious mixtures or slurries is that they have little or no form stability and they are molded into the final form by pouring the mixture into a space having externally supported boundaries or walls.
It is precisely because of this lack of moldability (which may be the result of poor workability and/or poor form stability), coupled with the low tensile strength per unit weight, that hydraulically settable materials have traditionally been useful only for applications where size and weight are not limiting factors and where the forces or loads exerted on the concrete are generally limited to compressive forces or loads, as in, e.g., roads, foundations, sidewalks, and walls.
Moreover, hydraulically settable materials have historically been brittle, rigid, unable to be folded or bent, and having low elasticity, deflection and flexural strength. The brittle nature and lack of tensile strength (about 1-4 MPa) in concrete is ubiquitously illustrated by the fact that concrete readily cracks or fractures upon the slightest amount of shrinkage or bending, unlike other materials such as metal, paper, plastic, or ceramic. Consequently, typical hydraulically settable materials have not been suitable for making small, lightweight objects, such as containers or thin sheets, which are better if made from materials with much higher flexibility and tensile strength per unit weight compared to typical hydraulically settable materials.
More recently, higher strength hydraulically settable materials have been developed which might be capable of being formed into smaller, denser objects. One such material is known as "Macro-defect Free" or "MDF" concrete, such as is disclosed in U.S. Pat. No. 4,410,366 to Birchall et al. See also, S. J. Weiss, E. M. Gartner & S. W. Tresouthick, "High Tensile Cement Pastes as a Low Energy Substitute for Metals, Plastics, Ceramics, and Wood," U.S. Department of Energy CTL Project CR7851-4330 (Final Report, November 1984). However, such high strength cementitious materials have been prohibitively expensive and would be unsuitable for making inexpensive sheets or containers where much cheaper materials better suited for such uses (e.g., paper and plastic) are readily available. Another drawback is that MDF concrete cannot be used to mass produce small, lightweight objects due to the high amount of time and effort involved in forming and hardening the material and the fact that it is highly water soluble. Therefore, MDF concrete has been limited to expensive objects of simple shape.
Another problem with traditional, and even more recently developed high strength, concretes has been the lengthy curing times almost universally required for most concretes. Typical concrete products formed from a flowable mixture require a hardening period of 10-24 hours before the concrete is mechanically self-supporting, and upwards of a month before the concrete reaches a substantial amount of its maximum strength. Extreme care has had to be used to avoid moving the hydraulically settable articles until they have obtained sufficient strength to be demolded. Movement or demolding prior to this time has usually resulted in cracks and flaws in the hydraulically settable structural matrix. Once self-supporting, the object could be demolded, although it has not typically attained the majority of its ultimate strength until days or even weeks later.
Since the molds used in forming hydraulically settable objects are generally reused in the production of concrete products and a substantial period of time is required for even minimal curing of the concrete, it has been difficult to economically and commercially mass produce hydraulically settable objects. Although zero slump concrete has been used to produce large, bulky objects (such as molded slabs, large pipes, or bricks which are immediately self-supporting) on an economically commercial scale, such production is only useful in producing objects at a rate of a few thousand per day. Such compositions and methods cannot be used to mass produce small, thin-walled objects at a rate of thousands per hour.
Demolding a hydraulically settable object can create further problems. As concrete cures, it tends to bond to the forms unless expensive releasing agents are used. It is often necessary to wedge the forms loose to remove them. Such wedging, if not done properly and carefully each time, often results in cracking or breakage around the edges of the structure. This problem further limits the ability to make thin-walled hydraulically settable articles or shapes other than flat slabs, particularly in any type of a commercial mass production.
If the bond between the outer wall of the molded hydraulically settable article and the mold is greater than the internal cohesive or tensile strengths of the molded article, removal of the mold will likely break the relatively weak walls or other structural features of the molded article. Hence, traditional hydraulically settable objects must be large in volume, as well as extraordinarily simple in shape, in order to avoid breakage during demolding (unless expensive releasing agents and other precautions are used).
Typical processing techniques of concrete also require that it be properly consolidated after it is placed in order to ensure that no voids exist between the forms or in the structural matrix. This is usually accomplished through various methods of vibration or poking. The problem with consolidating, however, is that extensive overvibration of the concrete after it has been placed can result in segregation or bleeding of the concrete.
"Bleeding" is the migration of water to the top surface of freshly placed concrete caused by the settling of the heavier aggregate. Excessive bleeding increases the water to cement ratio near the top surface of the concrete slab, which correspondingly weakens and reduces the durability of the surface of the slab. The overworking of concrete during the finishing process not only brings an excess of water to the surface, but also fine material, thereby resulting in subsequent surface defects.
For each of the foregoing reasons, as well as numerous others which cannot be listed here, hydraulically settable materials have not generally had application outside of the formation of large, slab-like objects, such as in buildings, foundations, walk-ways, or highways, or as mortar to adhere bricks or cured concrete blocks. It is completely counterintuitive, as well as contrary to human experience, to even imagine (let alone actually experience) the manufacture from hydraulically settable materials within the scope of the present invention of small lightweight sheets and other objects, which are comparable to lightweight sheets made from paper, cardboard, plastic, or polystyrene.
Yet, due to the more recent awareness of the tremendous environmental impact (not to mention the ever mounting political pressures) of using sheets made from paper, cardboard, plastic, polystyrene, and metals for a variety of single-use, mainly disposable, items such as containers or magazines, there has been an acute need (long since recognized to those skilled in the art) to find environmentally sound substitute materials such as hydraulically settable materials for these disposable items.
In spite of such pressures and long-felt need, the technology simply has not existed for the economic and feasible production of hydraulically settable materials which could be substituted for paper, cardboard, plastic, polystyrene, or metal sheets used to make a wide variety of disposable items such as containers. However, because hydraulically settable materials essentially comprise such environmentally neutral components such as rock, sand, clay, and water, they would be ideally suited from an ecological standpoint to replace paper, cardboard, plastic, or polystyrene materials as the material of choice for such applications.
B. The Impact of Paper, Cardboard, Plastic, Polystyrene, and Metals.
A huge variety of objects such as containers, packing materials, mats, disposable utensils, reading or other printed materials, and decorative items are presently mass-produced from paper, cardboard, plastic, polystyrene, and metals. The vast majority of such items eventually wind up within our ever diminishing landfills, or worse, are scattered on the ground or dumped into bodies of water as litter. Because plastic and polystyrene are essentially nonbiodegradable, they persist within the land and water as unsightly, value diminishing, and (in some cases) toxic foreign materials. Even paper or cardboard, believed by many to be biodegradable, can persist for years, even decades, within landfills where they are shielded from air, light, and water, which are all necessary for normal biodegration activities. Metal products utilize valuable natural resources in their manufacture, and if not recycled, remain in the landfill and unusable essentially forever.
In spite of the more recent attention that has been given to reduce the use of such materials, they continue to be used because of their superior properties of strength and, especially, mass productivity. Moreover, for any given use for which they were designed, such materials are relatively inexpensive, lightweight, easy to mold, strong, durable, and resistant to degradation during the use of the object in question.
Although each of these materials may be comparably priced to any of the other materials presently available, they are usually far more expensive than typical hydraulically settable materials. Because no rational business would ignore the economic benefit which would necessarily accrue from the substitution of radically cheaper hydraulically settable materials for paper, cardboard, plastic, polystyrene, or metal materials, the failure to do so can only be explained by a marked absence of available technology to make the switch.
In the manufacture of paper, the fiber slurry has upwards of 99% water which must be removed during the paper-making process. The slurry is sprayed onto a moving sieve bed through which water is extracted by a series of suction boxes. When the fiber sheet which is formed is removed from the moving bed, the fiber sheet still comprises 80% water. The fiber sheet then passes through a series of rollers which reduces the water content to about 50%; thereafter, heat is applied to dry the fiber sheet to form the paper product. This process, which has changed little in decades, is energy intensive, time consuming, and requires a significant initial investment.
The manufacturing processes of plastic sheets or products vary, but they typically require precise control of both temperature and shear stress in order to make a usable product. In addition, the typical polystyrene or plastic manufacturing process is a high consumer of energy. Similarly, manufacturing products from metals consumes high amounts of energy because of the elevated temperatures utilized in the processes, as well as requiring high shear stresses to fashion and mold the products. Of course, the initial capital investments for manufacturing processes utilizing metals are very high.
Recently, there has been a growing debate as to which of these materials (i.e., paper, plastic or metals) is more damaging to the environment. Consciousness-raising organizations have convinced many people to substitute one material for another in order to be more environmentally "correct." The debate often misses the point that each of these materials has its own unique environmental weaknesses. To one who is not fully informed, or who may lack an adequately rigorous scientific approach, one material may appear superior to another when viewed in light of a particular environmental problem, while ignoring different, often larger, problems associated with the supposedly preferred material.
In fact, paper, cardboard, plastic, polystyrene, and metal each has its own unique environmental weaknesses. The debate should, therefore, not be directed to which of these materials is more or less harmful to the environment, but should rather be directed toward asking: Can we find an alternative material which will solve most, if not all, of the various environmental problems associated with each of these presently used materials?
Based on the forgoing, what is needed are improved compositions and methods for manufacturing cementitious and other hydraulically settable sheets that can be formed into a variety of objects presently formed from paper, cardboard, polystyrene, plastic, or metal.
It would be a significant improvement in the art if such compositions and methods yielded hydraulically settable sheets which had properties similar to paper, cardboard, polystyrene, plastic, and metal. It would yet be a tremendous improvement in the art if such sheets could be formed into a variety of containers or other objects using the same or similar manufacturing apparatus and techniques as those presently used to form such objects from paper, cardboard, polystyrene, plastic, or metal.
It would yet be an important advancement in the art if such sheets did not result in the generation of wastes involved in the manufacture of paper, plastic, or metal materials. In addition, it would be a significant improvement in the art if such sheets, as well as containers or other objects made therefrom, were readily degradable into substances which are commonly found in the earth.
From a practical point of view, it would be a significant improvement if such materials and methods made possible the manufacture of sheets, containers, and other objects at a cost comparable or even superior to existing methods of manufacturing paper or polystyrene products. Specifically, it would be desirable to reduce the energy requirements and the initial capital investment costs for making products having the desirable characteristics of paper, plastic, and metals.
From a manufacturing perspective, it would be a significant advancement in the art of cement making to provide cementitious mixtures and methods for mass producing cementitious sheets which can rapidly be formed and substantially dried within a matter of minutes from the beginning of the manufacturing process.
Such materials and methods are disclosed and claimed herein.